Switched-mode power supplies (SMPS, also called switch- or switching-mode) are power supplies which transfer power from a source (e.g. mains power or a battery) to a load (e.g. a personal computer or a microchip) while converting voltage and current characteristics. SMPSs function by rapidly switching a section of a circuit comprising an energy storage (consisting of at least one inductor), a power source, a load, and a smoothing capacitor connected in parallel with the load. As the current through an inductor cannot change instantaneously, different configurations of the power source and energy storage can result in step-up behaviour (load voltage higher than source voltage) or step-down behaviour (load voltage lower than source voltage). The step-up or step-down ratio is determined by the characteristics of the energy storage, and the duty cycle of the switch.
For the examples considered below, the following relationships are useful. These assume that all inductors and capacitors are ideal, but the skilled person will appreciate how the calculations should be modified for non-ideal components. In the following equations, symbols subscripted with L refer to an inductor, symbols subscripted with C refer to a capacitor, E is the energy of the component, and all other symbols have their conventional meaning in electronics.
            V      L        =          L      ⁢                        ⅆ                      I            L                                    ⅆ          t                                E      L        =                  L        2            ⁢              (                  I          L          2                )                        E      c        =                  C        2            ⁢              (                  V          c          2                )            
An example SMPS is the “buck converter” shown in FIG. 1. The inductor L acts as the energy storage component and the capacitor C acts as a smoothing capacitor. Disregarding the smoothing capacitor, when the switch is closed there is a positive voltage across the inductor, and the current through the inductor increases. As a result, the voltage across the load is less than the source voltage by the inductor voltage. When the switch is opened current through the inductor begins to decrease, and there is a voltage across the inductor with the opposite polarity to the inductor voltage while the switch is open. Since the load and the terminals of the inductor are connected only by a diode, the voltage across the load is equal to the voltage across the inductor. Taking into account now the smoothing capacitor, this acts to prevent the voltage from changing too quickly, and ensures a closer approximation to steady DC voltage at the output. The diode in FIG. 1 may be replaced by a switch which always in the opposite state to the switch shown (i.e. when each switch is open, the other is closed).
The buck convertor is a step-down converter with step down ratio (during stable operation, i.e. assuming constant output load and assuming the inductor current does not drop to zero)
                    V        o                    V        i              =    D    ,where Vo is the output voltage, Vi is the input voltage, and D is the duty cycle of the switch.
Other SMPS topologies are known in the art, and the following description may be applied to them by the skilled person. The buck converter will be used in examples for the remainder of this disclosure, but the principles herein may be applied to other SMPS topologies unless indicated otherwise.
The current through the energy storage is not constant at any point, and therefore the output voltage is not constant, but will have some “ripple”, i.e. rising when the rising when the inductor current is higher than the load current and falling when it is lower than the load current. In the buck convertor, the amplitude of this ripple depends on the values of the inductor and the smoothing capacitor. In a generalised convertor, the amplitude depends at least on the inductance of the energy storage and the capacitance of the smoothing capacitor, but may depend upon other components/values. The bigger either or both of the inductance and capacitance are, the lower the amplitude of the ripple and the smoother the output.
Increasing the inductance and/or capacitance will however decrease the rate at which the circuit can respond to changes in the load. For a buck converter, if the load current drops suddenly, the output voltage will rise sharply and vice versa, as shown in FIG. 2. This spike in output voltage will decay back to the target output voltage over time. The decay tie may be shortened by modifying the duty cycle of the SMPS in dependence upon the output voltage or current.
A voltage control circuit is shown in FIG. 2. The output voltage is compared to a reference voltage (Vref), and the error amplified. This error is then compared to a ramp signal, such that when the error is greater than the ramp signal the output is a logical “high”, and when the error is less than the ramp signal the output is logical “low”. The output from the comparator creates the pulse width modulation which controls the switching of the SMPS. The ramp signal and reference voltage are configured such that the output voltage of the SMPS tends towards the target output voltage.
A current control circuit is shown in FIG. 3. The circuit is similar to that of the voltage control circuit, except that a voltage Vindcurrent is added at the comparator stage. Vindcurrent is dependent on the inductor current, which allows for faster response of the feedback circuit. The inductor current may be measured by directly integrating the voltage across the inductor, though this is likely to drift. Alternatively, the voltage across the switch S1 may be measured (since the current through S1 is equal to the current through the inductor while the switch is closed). The current/voltage relationship of S1 varies significantly with process, voltage, and temperature, and so, to ensure a more accurate measurement, a matched smaller transistor may be used, controlled such that it has the same drain voltage as S1, resulting in a current that is proportional to the switch current in S1. The matched transistor may be controlled more tightly for other variables. A downside of current control is that the current measurement introduces a considerable amount of noise into the control, and hence into the output voltage.
For either control method, if the rate of change of the duty cycle is too high, then there will be stability problems, and the larger the inductance and/or the capacitance are for the SMPS, the slower the rate of change of the duty cycle must be. However, if the rate of change of the duty cycle is lower, then the voltage will overshoot or undershoot the desired voltage by a larger amount.
FIG. 4 shows the output voltage (upper graph) and load (lower graph) of a voltage controlled SMPS, and FIG. 5 shows the output voltage (upper graph) and load (lower graph) of a current controlled SMPS. Other than the feedback circuit, the SMPSs are identical. It is clear that the current controlled SMPS can cope better with changes in output load than the voltage controlled SMPS. It is also clear however that there is still scope for improvement.